The Web Authentication API is an extension of the Credential Management API that enables public key authentication, enabling password-less authentication and / or secure second-factor authentication without SMS texts.

Web authentication concepts and usage

The Web Authtentication API (also referred to as WebAuthn) uses asymmetric (public-key) cryptography instead of passwords or SMS texts for registering, authenticating, and second-factor authentication with websites. This resolves significant security problems related to phishing, data breaches, and attacks against SMS texts or other second-factor authentication methods while at the same time significantly increasing ease of use (since users don't have to manage dozens of increasingly complicated passwords).

Many websites already have pages that allow users to register new accounts or sign in to an existing account, and WebAuthn acts as a replacement or suppliment to those on those existing webpages. Similar to the other forms of the Credential Management API, the Web Authentication API has two basic methods that correspond to register and login:

navigator.credentials.create() - when used with the publicKey option, creates new credentials, either for registering a new account or for associating a new asymmetric key pair credentials with an existing account.

navigator.credentials.get() - when used with the publicKey option, uses an existing set of credentials to authenticate to a service, either logging a user in or as a form of second-factor authentication.

Please note: both create() and get() require a Secure Context (e.g. - the server is connected by https or is the localhost), and will not be available for use if the browser is not operating in a secure context.

In their most basic forms, both create() and get() receive a very large random number called a challenge from the server and they return the challenge signed by the asymmetric key pair back to the server. This proves to the server that a user is in possession of the key pair required for authentication without revealing any secrets over the network.

In order to understand how the create() and get() methods fit into the bigger picture, it is important to understand that they sit between two components that are outside the browser:

Server - the WebAuthn API is intended to register new credentials on a server (also referred to as a service or a relying party) and later use those same credentials on that same server to authenticate a user.

Authenticator - the credentials are created and stored in a device called an authenticator. This is a new concept in authentication: when authenticating using passwords, the password is stored in a user's brain and no other device is needed; when authenticating using WebAuthn, the password is replaced with a key pair that is stored in an authenticator. The authenticator may be embedded into an operating system, such as Windows Hello, or may be a physical token, such as a USB or Bluetooth Security Key.

Registration

A typical registration process has six steps, as illustrated in Figure 1 and described further below. This is a simplification of the data required for the registration process that is only intended to provide an overview. The full set of required fields, optional fields, and their meanings for creating a registration request can be found in the PublicKeyCredentialCreationOptions dictionary. Likewise, the full set of response fields can be found in the PublicKeyCredential interface (where PublicKeyCredential.response is the AuthenticatorAttestationResponse interface). Note most JavaScript programmers that are creating an application will only really care about steps 1 and 5 where the create() function is called and subsequently returns; however, steps 2, 3, and 4 are essential to understanding the processing that takes place in the browser and authenticator and what the resulting data means.

Figure 1 - a diagram showing the sequence of actions for a WebAuthn registration and the essential data associated with each action.

The registration steps are:

Application Requests Registration - The application makes the initial registration request. The protocol and format of this request is outside of the scope of WebAuthn.

Server Sends Challenge, User Info, and Relying Party Info - The server sends a challenge, user information, and relying party information to the JavaScript program. The protocol for communicating with the server is not specified and is outside of the scope of WebAuthn. Typically, server communications would be REST over https (probably using XMLHttpRequest or Fetch), but they could also be SOAP, RFC 2549 or nearly any other protocol provided that the protocol is secure. The parameters received from the server will be passed to the create() call, typically with little or no modification and returns a Promise that will resolve to a PublicKeyCredential containing an AuthenticatorAttestationResponse. Note that it is absolutely critical that the challenge be a large buffer of random information (e.g. - more than 100 bytes) and it MUST be generated on the server in order to ensure the security of the registration process.

Browser Calls authenticatorMakeCredential() on Authenticator - Internally, the browser will validate the parameters and fill in any defaults, which become the AuthenticatorResponse.clientDataJSON. One of the most important parameters is the origin, which recorded as part of the clientData so that the origin can be verified by the server later. The parameters to the create() call are passed to the authenticator, along with a SHA-256 hash of the clientDataJSON (only a hash is sent because the link to the authenticator may be a low-bandwidth NFC or Bluetooth link and the authenticator is just going to sign over the hash to ensure that it isn't tampered with).

Authenticator Creates New Key Pair and Attestation - Before doing anything, the authenticator will typically ask for some form of user verficiation. This could be entering a PIN, using a fingerprint, doing an iris scan, etc. to prove that the user is present and consenting of the registration. After the user verification, the authenticator will create a new asymmetric key pair and safely store the private key for future reference. The public key will become part of the attestation, which the authtenticator will sign over with a private key that was burned into the authenticator during its manufacturing process and that has a certificate chain that can be validated back to a root of trust.

Authenticator Returns Data to Browser - The new public key, a globally unique credential id, and other attestation data are returned to the browser where they become the attestationObject.

Server Validates and Finalizes Registration - Finally, the server is required to perform a series of checks to ensure that the registration was complete and not tampered with. These include:

Verifying that the challenge is the same as the challenge that was sent

Ensuring that the origin was the origin expected

Validating that the signature over the clientDataHash and the attestation using the certificate chain for that specific model of authenticator

A complete list of validation steps can be found in the WebAuthn specification. Assuming that the checks pan out, the server will store the new public key associated with the user's account for future use -- that is, whenever the user desires to use the public key for authentication.

Authentication

After a user has registered with WebAuthn, they can subsequently authenticate (a.k.a. - login or sign-in) with the service. The authentication flow looks similar to the registration flow, and the illustration of actions in Figure 2 may be recognizable as being similar to the illustration of registration actions in Figure 1. The primary differences between registration and authentication are that: 1) authentication doesn't require user or relying party information; and 2) authentication creates an assertion using the previously generated key pair for the service rather than creating an attestation with the key pair that was burned into the authenticator during manufacturing. Again, the discription of authentication below is a broad overview rather than getting into all the options and features of WebAuthn. The specific options for authenticating can be found in the PublicKeyCredentialRequestOptions dictionary, and the resulting data can be found in the PublicKeyCredential interface (where PublicKeyCredential.response is the AuthenticatorAssertionResponse interface) .

Figure 2 - similar to Figure 1, a diagram showing the sequence of actions for a WebAuthn authentication and the essential data associated with each action.

Application Requests Authentication - The application makes the initial authentication request. The protocol and format of this request is outside of the scope of WebAuthn.

Server Sends Challenge - The server sends a challenge JavaScript program. The protocol for communicating with the server is not specified and is outside of the scope of WebAuthn. Typically, server communications would be REST over https (probably using XMLHttpRequest or Fetch), but they could also be SOAP, RFC 2549 or nearly any other protocol provided that the protocol is secure. The parameters received from the server will be passed to the get() call, typically with little or no modification. Note that it is absolutely critical that the challenge be a large buffer of random information (e.g. - more than 100 bytes) and it MUST be generated on the server in order to ensure the security of the authentication process.

Browser Call authenticatorGetCredential() on Authenticator - Internally, the browser will validate the parameters and fill in any defaults, which become the AuthenticatorResponse.clientDataJSON. One of the most important parameters is the origin, which recorded as part of the clientData so that the origin can be verified by the server later. The parameters to the create() call are passed to the authenticator, along with a SHA-256 hash of the clientDataJSON (only a hash is sent because the link to the authenticator may be a low-bandwidth NFC or Bluetooth link and the authenticator is just going to sign over the hash to ensure that it isn't tampered with).

Authenticator Creates an Assertion - The authenticator finds a credential for this service that matches the Relying Party ID and prompts a user to consent to the authentication. Assuming both of those steps are successful, the authenticator will create a new assertion by signing over the clientDataHash and authenticatorData with the private key generated for this account during the registration call.

Authenticator Returns Data to Browser - The authenticator returns the authenticatorData and assertion signature back to the browser.

Interfaces

WebAuthn extends the Credential Management API's create() and get() methods to take a new option: publicKey. When the publicKey option is passed to create() and / or get(), the Credential Management API will create a new public key pair or get an authentication for a key pair, respectively.

Part of the PublicKeyCredential, the AuthenticatorResponse includes information from the browser (such as the challenge and origin); as well as information from the authenticator such as an AuthenticatorAttestationResponse (for new credentials) or an AuthenticatorAssertionResponse (when authenticating with existing credentials).

When a PublicKeyCredential has been created with the create() call, it will include an AuthenticatorAttestationResponse. This is the authenticator's way of providing a cryptographic root of trust for the new key pair that has been generated.

When a PublicKeyCredential is the result of a get() call, it will include an AuthenticatorAssertionResponse. This is the authenticator's way of proving to a service that it has the key pair and that the authentication request is valid and approved.